RFR: Add Repository::abortMerge

[Erik Duveblad via github.com]

Hi all,

this patch add `Repository::abortMerge` and a corresponding unit test.

## Testing
- [x] `sh gradlew test`
- [x] Added new unit test

Thanks,
Erik

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Commits:
 - fee82c70:	Add Repository::abortMerge

Pull request:
https://git.openjdk.java.net/skara/pull/101

Webrev:
https://webrevs.openjdk.java.net/skara/101/webrev.00

Patch:
https://git.openjdk.java.net/skara/pull/101.diff

Fetch command:
git fetch https://git.openjdk.java.net/skara pull/101/head:pull/101